1. Field of the Invention
The invention, in general, relates to a power train for automotive vehicles and, more particularly, to a drive unit thereof provided with an input shaft and at least one output shaft in which a hydrostatic displacement engine connected to a stationary fluid reservoir by a suction tube generates pressure for reducing any differences in rotation s between the input and output shafts by actuating a hydraulic coupling.
2. The Prior Art
Many different drive units of the general kind have become known. They differ from each other in their construction as well as in the way in which they function to reduce the difference in rotations. For instance, U.S. Pat. Nos. 5,611,746; 5,595,214 and 5,536,215 disclose trochoidal pumps. U.S. Pat. Nos. 4,091,901; 4,727,966 and 3,923,113 teach gear pumps; and the apparatus disclosed by U.S. Pat. No. 4,730,514 makes use of a vane pump. Accordingly, they are each equipped with an appropriately structured pair of components which are movable relative to each other.
In every instant, pressure is generated whenever there is a difference in rotation between those two components which reduces the difference either by way of a friction clutch as in the case of the embodiments shown in FIGS. 1 and 22 of International Application WO 95/23931 or by impeding the relative movement between the two components as in the embodiment depicted in FIG. 12 of this publication.
Where such a drive unit is equipped with two output shafts, a differential transmission may be provided between them (WO 95/23931 FIGS. 1 and 12) in which the differential effect is blocked by the built-up pressure. But as disclosed by U.S. Pat. No. 4,091,901, a similar effect may be achieved where there is no differential transmission.
Such drive units are used in the power train of automotive vehicles for distributing drive torque to the wheels mounted on one axle, or to two axles, as the case may be. In case one or more wheels loose contact with the driving surface, which results in an increasing difference in rotations, such drive units change the torque distribution such that traction is provided to those wheels only which have not lost surface contact.
Problems inherent in such arrangements stem from rotational differences resulting from circumstances other than the loss of surface contact, such as, for instance, driving in curves, braking, towing or driving with an emergency wheel. The changes in the torque distribution resulting therefrom are similarly undesirable and, in many cases, dangerous. To overcome these problems, it is customary additionally to provide a freewheeling clutch which must, however, be bypassed when driving in reverse, for instance.
U.S. Pat. No. 4,727,966 discloses a drive unit of the kind referred to in which the operating fluid for the displacement pump is pumped from a stationary housing. To control the coupling action and, more particularly, to limit the coupling action as a function of operating parameters, fluidally adjustable pressure-relief valves are provided at the pressure side of the displacement pump. Since the pressure chambers are provided in rotating parts of the displacement pump, actuation of the control members in the rotating parts and the supply of operating fluid thereinto requires a rotating input. Not only does this entail a complex structure, but the control is also prone to malfunction and is inaccurate. Moreover, the responsiveness of a pressure-relief valve leads to a strongly retarded reduction of the coupling action in view of the fact that the high flow rate at high pressure has to be accommodated by the narrow passage of the pressure-relief valve. Hence, the valve constitutes a throttle which does not allow a complete pressure reduction.